I'm considering using FileVault 2 when I upgrade to Lion. Though my understanding of it is weak, it seems as though it shouldn't really increase the number of reads or writes per disk operation compared to a non-encrypted drive.

I don't even know if there's any kind of encryption that would increase reads/writes dramatically, since all decryption presumably happens in memory. But it seemed like something I ought to ask about.

2 Answers
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Encryption happens on-the-fly. If the data was written to the hard drive, then passed through an encryption algorithm that then deleted the written data and re-wrote it, encrypted, that would be hella inefficient, and would largely defeat the purpose. I think the answer to your question is no, but of course remember that if you're going from a data set that's not yet encrypted, then all of that data needs to be processed, so that initial process of reading, encrypting and re-writing data will have an effect.

If initial encryption takes longer than expected, bear in mind that probably the entire volume (not just the used space) must be converted. During conversion, for more detail than is given in System Preferences, run the following command in Terminal and in the results, focus on the sizes: diskutil coreStorage list
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Graham PerrinAug 2 '11 at 9:32

Depends on your SSD. Some (e.g. those with SandForce controllers) use compression to lower the number of writes on the SSD. Using encryption will make compressible data incompressible, resulting in a larger number of writes to the SSD, and consequently will wear it out faster.

Of course, the initial encryption will also generate writes (about one cycle for each block on the drive), but it's probably nothing to worry about considering that most SSDs are rated for 5000 write cycles.

10.6 added filesystem level compression, so you can compress, then encrypt your data. I haven't tried turning it manually, but I have used Squeezer (formerly known as Squeeze) to turn it on.
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Will RossFeb 25 '12 at 2:54